I have recently started an observing program to study the distribution of dynamically old giant stars in the disk, looking for (i) large amplitude spiral arms and (ii) small amplitude corrugations.

1. (i) Theoretical studies of spiral density waves usually assume that the mass density contrast is weak and the waves linear. Recent observations in red and near-IR light (e.g., Elmegreen and Elmegreen 1984; Rix and Zaritsky 1994), coupled with stellar population models, suggest otherwise. In the Galaxy, we can compare observations of diffuse NIR light and of individual stars, eliminating the need for population modeling. Moreover, we can estimate distances to stars to learn about 3D structure.

2. (ii) Recent work by Malhotra (1994a; 1994b) has shown that the midplane of the molecular and atomic gas differs from the nominal (Galactic latitude b = 0°) plane by up to 50 pc in the inner Galaxy. Various observations (Djorgovski & Sosin 1989, Freudenreich et al 1994, Carney & Seltzer 1993) show that the stellar distribution also deviates from b = 0. We will choose fields to test the possibility that the stars behave like the gas. This constrains theories of the corrugations; e.g., if old stars participate then magnetic fields aren't the cause.